1. Field of the Invention
The present invention relates to metal casting methods utilizing sand molding processes, and more particularly to a sand molding process which includes a method for providing pre-shaped heat exchange piping in the sand mold before casting of the metal thereinto. The present invention is specifically directed to a method for predictably and successfully accommodating heat expansion of the aforesaid piping in response to casting of the metal into the sand mold.
2. Description of the Prior Art
It is well known in the art to cast metals in a predetermined shape through the use of sand molding techniques. Typically, the conventional sand molding process utilizes a pattern which is used to shape a sand mold that is then used to define the shape of the cast metal. The type of pattern used may be disposable or permanent, the use of one or the other defining which steps are to be followed in the casting process.
The typical steps followed when a disposable pattern is used are: A disposable pattern is fabricated to specification; the pattern is typically formed out of the polystyrene family of materials, foundry grade polystyrene being preferred. The pattern is covered by a refractory coating and placed on a molding board. A box known as a "drag flask" is placed on the molding board. Foundry sand is poured into the drag flask, during which it is rammed around the outer edge of the pattern and rammed generally inside the drag flask. Before the drag flask is filled, a "gate" is provided by positioning an appropriately shaped polystyrene material adjacent the disposable pattern. The drag flask is filled and the sand leveled off. A bottom board is now placed on the top of the flask. The drag flask is then inverted so as to rest on the bottom board. The molding board is removed and a "cope flask" is placed above the drag flask. To provide passage of the molten casting metal into the pattern, a "sprue" is provided in the cope flask by insertion of an appropriately shaped polystyrene material adjacent the gate. Next, a "riser" is provided in the cope flask to accommodate contraction of the molten casting metal by placement of an appropriately shaped polystyrene material adjacent the disposable pattern. Foundry sand is then poured into the cope flask and rammed as described above. The mold is now ready for casting metal in the shape of the pattern by introduction of molten casting metal into the sprue.
The typical steps followed when a permanent pattern is used are: A permanent pattern is fabricated to specification; the pattern is typically made of wood, but may also be made of other durable materials, such as metal, plastic, plaster or clay. The pattern is placed on a molding board. A drag flask is then placed on the molding board. Parting compound is dusted over the pattern. Foundry sand is poured into the drag flask and rammed about the pattern edges and rammed generally within the drag flask. The foundry sand is leveled off and a bottom board placed over the drag flask. The drag flask is then inverted so as to rest on the bottom board. The molding board is removed and a cope flask placed on the drag flask. A riser pin and a sprue pin are provided in the cope flask adjacent the pattern. Foundry sand is poured into the cope flask and rammed as described above. The pins are removed from the cope portion of the mold and the cope is then removed and carefully set down elsewhere. The pattern is gently lifted from the drag portion of the mold, and the cavity resulting therefrom is covered by a refractory coating. A gate is cut in the drag portion of the mold so as to connect with the sprue of the cope portion of the mold. The cope is now replaced above the drag. The mold is now ready for casting metal in the shape of the pattern by introduction of molten casting metal into the sprue.
While these processes are very successful for the casting of metals, they do not inherently provide for heat exchange channels in the metal casting. Frequently, however, the metal casting must be provided with heat exchange channels which run proximately with the surface of the metal casting. This requirement may arise, for example, when the metal casting is to be used as a die for plastic injection molding. Conventionally, these channels are provided by boring or routing into the metal casting subsequent to its casting in the sand mold. Examples of this technique are disclosed by Summers U.S. Pat. No. 3,572,420, Auman et al U.S. Pat. No. 3,763,920 and Alberny U.S. Pat. No. 4,009,749. Alternatively, an external heat exchange jacket may provided which surrounds the metal casting. Examples of this latter technique are disclosed in Wertli U.S. Pat. No. 3,530,926, Adamec et al U.S. Pat. No. 3,592,259 and Sevastakis U.S. Pat. No. 4,493,361. In either case, these provisions for heat exchange channeling for the metal casting are very expensive and labor intensive. What is needed is a method for providing heat exchange channels at the time that the metal casting is being cast.
The present inventor has been in the foundry business for many years and has long been engaged in seeking a reliable, predictable and successful method of casting metals with integral heat exchange piping. In an article published in the periodical Plastics Machinery & Equipment, Vol. 14, No. 11, pages 42 and 43 (Nov. 1985), entitled "Casting Technique Reduces Costs of Molding", an early, experimental method developed by the inventor is disclosed. In this method a foam pattern is used. The method is as follows:
" . . . The foam pattern is placed in a chemically bonded sand mold. The sand chemically joins itself around the pattern, and then seats into the exact dimensions of the foam pattern. Schwarb Foundry's proprietary metal is heated in a furnace and then poured over the foam mold, the pattern evaporates into gas, and the mold cavity is filled with metal. . . . The metal then cools, forming the cavity or core of the cast mold.
"Steel pipes for heating or cooling are positioned within the foam pattern. The molten metal is poured over the pipes without melting them. As the metal cools, it solidifies around the pipes, providing cooling lines in the cast mold. . . .
"The key to this casting technique lies in two factors: first, the composition of the metal and, second, the pouring technique, both proprietary. The purported savings realized in cast molds over conventionally made molds lies in the amount of purchased metal to be machined. In conventional moldmaking, two solid blocks of P20 or similar steel are used to make the cavity and the core. Deep-draw cavities require significant amount of stock removal. The cast molds require matching off only 0.375 to 0.5 inch of steel to prepare the mold. . . . "
However, while the concept of including heat exchange piping within the pattern cavity prior to the casting step was disclosed in this article, there was no mention therein of how to deal with unpredictable and uncontrolled expansion movements of the piping when the molten casting metal came into contact with it. This severe and debilitating problem was addressed by the inventor in several ways before arriving at the method disclosed by the present invention.
One method proffered by the inventor was to control heating of the pipes by blowing a high volume of air through the pipes during the casting step. This proved to be workable, but there was still unpredictable pipe movement and the air flow system was costly to install and operate.
What is needed, therefore, is a method for casting metals utilizing sand mold techniques by which heat exchange piping may be provided contemporaneously with the casting step and heat expansion movements of the aforesaid piping during the casting step may be predictably accommodated with a minimum of time and expense.